1. Field of the Invention
The present invention relates to an apparatus for concealing an error of an orthogonal transform coding which is adapted to conceal an error of an image data read out when the coded image data is decoded.
2. Description of the Related Art
In general, for efficiently recording a digital moving pictures on a magnetic tape or a disk or transmitting it on a line, the moving pictures data is efficiently coded for reducing the quantity of information to be recorded or transmitted.
The inventors of the present application know a method, which has been proposed in the Japanese Patent Application No. 3-11882 by some of the inventors of the present application, in order to efficiently code the moving pictures data.
The above-mentioned method is arranged to divide one-screen data, that is, one-frame data into several blocks, each block consisting of 8-by-8 pixels, orthogonally transform each of those blocks, and quantize each transform coefficient in the light of the statistical quality of the block.
After quantizing the transform coefficients, the quantized data and the additional information accompanied with it are error-correcting coded if any. The data which is error-correcting coded data is recorded on a recording medium.
In order to read the data recorded by the foregoing method, there may be a value to be erroneously read out. For the error-corrected code, when it is read out, it is decoded in an error-correcting manner. Then the location of some erroneous data is detected. Further, some of error detected data is corrected.
However, there may be left some pieces of data error-detected but non-corrected. That is, those pieces of data are found to be erroneous but the correct values about those pieces of data are not found out. If the moving pictures data is decoded with the erroneous data included therein, the reproduced image is made inferior in quality.
Against the data whose error is found but not corrected, the error modification is performed so as to suppress the erroneous values by using the correlation in the frame.
As a first known method for concealing an error on the reproduced image, the actual pixel values about an erroneous pixel is measured from the pixels around the erroneous pixel and the measured pixel values are replaced with the erroneous pixel. For example, as shown in FIG. 1, the reference alphabets a to f denote pixel values, respectively. If a value of e is found to be erroneous after the error detection is done, as indicated in the following expression (1), a value of e is measured from the pixels around the erroneous value of e. The erroneous value of e is replaced with the measured value of e'. EQU e'=(b+d)/2 (1)
As a second known method for concealing an error, as disclosed in the Japanese Patent Laying Open No. 61-147690, at first, the image is broken into several blocks. For each block, an additional code and a coded data are derived. The additional code consists of a minimum value of the pixel values of the block and a dynamic range about all the pixel values within the block. The coded data is formed by quantizing the difference between each pixel values and a minimum value of the pixel values of the block. If the additional code is found to be erroneous, the average value of the additional coded values around the erroneous coded data is derived as a measured value and the erroneous coded data is replaced with the measured value.
In the case of using the transform coding method, an error takes place not in a specific pixel values but a transform coefficient. If the inverse transformation is performed with respect to the block data including the erroneous transform coefficient, the error gives an adverse effect on the overall block. Hence, all the pixel values included in the block are all made erroneous on the reproduced image.
In the first known method for concealing an error, a real value is measured from the pixel values around the erroneous pixel. Therefore, if the pixel values around the erroneous pixel is properly reproduced, this first known method is effective. However, if the pixels around the erroneous pixel include one or more erroneous ones, this method is not properly effective. For example, consider that the pixels of a to f shown in FIG. 1 are all erroneous. If a value of e is measured from the pixels around the pixel of e by means of the expression (1) for concealing the value of e, the measuring accuracy is degraded because the values of b and d used for measuring the value of e are erroneous. That is, the first known concealing method is not effective if some or all the pixels on the block are erroneous.
The foregoing second known concealing method is arranged to measure the additional information greatly influencing the overall block such as a minimum value of the pixel values of one block and a dynamic range about all the pixels of one block by using the correlation among the block and its adjacent blocks. If the correlation among the error-included block and its adjacent blocks is large, the second known concealing method is effective. However, if the correlation is small, the modified pixel values on the overall block may be far away from the real pixel values.
As another disadvantage, if the erroneous pixel values(s) greatly influence(s) the overall block, a slight difference between the measured value(s) and the real value(s) brings about a visually large defect on the reproduced image. Further, since the average value is used as the measured value, the overall image become disadvantageously dull.